Under the recent circumstances where energy saving is promoted, in the electrophotographic technical field, there has been an attempt to reduce the energy (that is, to achieve low temperature fixing) in a fixing device for the purpose of a decrease in power consumption and an increase in printing speed.
However, as such low temperature fixing is promoted, the thermal stability of a toner used has been reduced. Accordingly, there has been a problem in that heat-resistant storage properties cannot be sufficiently obtained during storage and transportation. Also, there has been another problem in that toner components such as colorants or parting agents are exposed on the surface of a toner, and therefore stable charging properties cannot be maintained for a long period of time.
A known solution to the above-described problems is to use a toner having a structure configured by covering a toner surface with a resin, a so-called core-shell structure (for example, see Patent Literatures 1 and 2).
In general, a toner produced by an emulsion association method includes associated particles obtained by aggregating and fusing resin fine particles and fine particles of a toner component such as a colorant using an aggregating agent such as metal salts. In this case, a large amount of an aggregating agent is required. Therefore, particularly when the resin fine particles include a monomer containing a polar group such as a carboxylic acid group, metal salts, which are derived from the aggregating agent, attached to or contained in the associated particles are hardly removed completely by a washing treatment. Furthermore, since the metal salts derived from an aggregating agent have high hygroscopicity, a toner obtained has also high hygroscopicity. As a result, when image formation is performed in a high temperature and high humidity environment, the charging ability of a toner is lowered compared with the case when performed in a low temperature and low humidity environment.
When a core-shell structure is formed in such an emulsion association-type toner, it is desirable that the toner surface is covered with a resin without using an aggregating agent as far as possible.
For example, Patent Literatures 3 and 4 disclose a technology of defining the content of a divalent or trivalent metal element and the content of a polyvalent metal element in view of an influence of metal salts. However, the improvement effect is not sufficient. Thus, further improvement, including a resin, is being required with respect to low temperature fixability and controlled environment variation difference in a charging ability.